Speedo driven gear. which one?
Thread Starter
Safety Car
Joined: Oct 2006
Posts: 3,993
Likes: 9
From: Gig Harbor WA
Speedo driven gear. which one?
Anybody know the STOCK driven gear tooth count in my 72 sbc/auto/3.08 speedo??
Isn't the DRIVE gear in the tranny, and the DRIVEN gear in the speedo?
I swapped to a 2004R and the websites call for a 13 tooth (white) drive gear, and a 30 tooth (blue) driven gear..
Thanks,
Rob
Isn't the DRIVE gear in the tranny, and the DRIVEN gear in the speedo?
I swapped to a 2004R and the websites call for a 13 tooth (white) drive gear, and a 30 tooth (blue) driven gear..
Thanks,
Rob
Thread Starter
Safety Car
Joined: Oct 2006
Posts: 3,993
Likes: 9
From: Gig Harbor WA
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
The "drive" and "driven" speedo gears are both in the tranny. The drive gear is a [sleeve-type] worm gear that is placed on the output shaft; the driven gear is placed in the small bore at the top/back of the tranny and it meshes with the drive gear...plus, it receives the speedo cable which is then screwed down to the gear carrier.
As far as determining what speedo driven gear you need...that depends on what drive gear is in the tranny. If there is a gear set in there now, hook it up and drive it on the highway at 60 mph; time each mile and average it to the number of seconds per mile. For every 3 second difference from a time of one minute [ 60 seconds per mile traveled], there needs to be a one tooth difference in the drive gear. A shorter time (less than 60 seconds) requires more teeth; a longer time requires fewer teeth. So, if you are running at 60 mph and it takes you 54 seconds to go exactly one mile, you need to get a gear with 2 more teeth than the drive gear you presently have. Hope that helps figure it out.
As far as determining what speedo driven gear you need...that depends on what drive gear is in the tranny. If there is a gear set in there now, hook it up and drive it on the highway at 60 mph; time each mile and average it to the number of seconds per mile. For every 3 second difference from a time of one minute [ 60 seconds per mile traveled], there needs to be a one tooth difference in the drive gear. A shorter time (less than 60 seconds) requires more teeth; a longer time requires fewer teeth. So, if you are running at 60 mph and it takes you 54 seconds to go exactly one mile, you need to get a gear with 2 more teeth than the drive gear you presently have. Hope that helps figure it out.
Thread Starter
Safety Car
Joined: Oct 2006
Posts: 3,993
Likes: 9
From: Gig Harbor WA
The "drive" and "driven" speedo gears are both in the tranny. The drive gear is a [sleeve-type] worm gear that is placed on the output shaft; the driven gear is placed in the small bore at the top/back of the tranny and it meshes with the drive gear...plus, it receives the speedo cable which is then screwed down to the gear carrier.
As far as determining what speedo driven gear you need...that depends on what drive gear is in the tranny. If there is a gear set in there now, hook it up and drive it on the highway at 60 mph; time each mile and average it to the number of seconds per mile. For every 3 second difference from a time of one minute [ 60 seconds per mile traveled], there needs to be a one tooth difference in the drive gear. A shorter time (less than 60 seconds) requires more teeth; a longer time requires fewer teeth. So, if you are running at 60 mph and it takes you 54 seconds to go exactly one mile, you need to get a gear with 2 more teeth than the drive gear you presently have. Hope that helps figure it out.
As far as determining what speedo driven gear you need...that depends on what drive gear is in the tranny. If there is a gear set in there now, hook it up and drive it on the highway at 60 mph; time each mile and average it to the number of seconds per mile. For every 3 second difference from a time of one minute [ 60 seconds per mile traveled], there needs to be a one tooth difference in the drive gear. A shorter time (less than 60 seconds) requires more teeth; a longer time requires fewer teeth. So, if you are running at 60 mph and it takes you 54 seconds to go exactly one mile, you need to get a gear with 2 more teeth than the drive gear you presently have. Hope that helps figure it out.
Are the gears accessible without dropping the tranny pan??
Thanks again!
Rob
Thread Starter
Safety Car
Joined: Oct 2006
Posts: 3,993
Likes: 9
From: Gig Harbor WA
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Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
You shouldn't need to change the drive gear. Just buy the correct match for whatever drive gear you have to get the right ratio for the speedo. There are more than one combination of gears available for a 3.08 gear set.
Burning Brakes
Joined: Nov 2006
Posts: 893
Likes: 7
From: Orange Park Florida
If you have a TH400, you can remove the speedo cable and the driven gear housing and colored gear. Look inside the tranny at the drive gear, mark a spline, have someone turn a rear wheel one full turn and count the number of splines. A good tranny repair shop when given the drive gear size (number of splines), rear end ratio, and tire size can tell you what color gear to use.
3rd Gear
Joined: Sep 2012
Posts: 3
Likes: 0
Lower the drive line end from the trans, take the rear housing off the trans, R&R the driven gear. It's that simple. You may need to R&R the sleeve if the gear is too large, but in your case, I do not think so.
The Best to you,
I need a 33 tooth driven gear to compensate for my 3.42 rear axle.
Can anyone help me to locate this gear?
I've tried many places and sites.
I ordered one in stock from GM Parts Direct that they had, but was cancelled when it became unavailable (in 5 days).
If I find it, I'll share the location.
My approximate original setup
Rear axle gear ratio 2.72
Tire diameter in inches 26.40
Ratio Required 0.4817 (13 x 27 gear set => 13/27 = .4814)
My new setup
Rear axle gear ratio 3.42
Tire diameter in inches 27.40
Ratio Required 0.3976
The following are my Preferred Gear Combinations
Drive Driven Ratio
10 25 0.4000
11 28 0.3929
12 20 0.4000
13 33 0.3939 (my original was a 13 x 27 set)
14 34 0.4000
15 38 0.3947
16 41 0.3902
17 43 0.3953
18 45 0.4000
I can therefore, utilize most any one of these gear combination.
The least costly would be the 33 driven gear, if I can find it.
These gears are getting really hard to find.
Thanks,
JamesPDX
Portland, Oregon
The Best to you,
I need a 33 tooth driven gear to compensate for my 3.42 rear axle.
Can anyone help me to locate this gear?
I've tried many places and sites.
I ordered one in stock from GM Parts Direct that they had, but was cancelled when it became unavailable (in 5 days).
If I find it, I'll share the location.
My approximate original setup
Rear axle gear ratio 2.72
Tire diameter in inches 26.40
Ratio Required 0.4817 (13 x 27 gear set => 13/27 = .4814)
My new setup
Rear axle gear ratio 3.42
Tire diameter in inches 27.40
Ratio Required 0.3976
The following are my Preferred Gear Combinations
Drive Driven Ratio
10 25 0.4000
11 28 0.3929
12 20 0.4000
13 33 0.3939 (my original was a 13 x 27 set)
14 34 0.4000
15 38 0.3947
16 41 0.3902
17 43 0.3953
18 45 0.4000
I can therefore, utilize most any one of these gear combination.
The least costly would be the 33 driven gear, if I can find it.
These gears are getting really hard to find.
Thanks,
JamesPDX
Portland, Oregon
Race Director
Joined: Apr 2011
Posts: 11,183
Likes: 3,336
From: North of Toronto - Ontario
I went to the local transmission shop (private shop, not a chain, been around forever)
I needed a 43 driven, the guy goes into the back storage room and a few minutes later comes out with a virtually new gear, I think he charged me $5
Check your local trans shops, walking in might be better than phone-calls
HIH
Mooser
I needed a 43 driven, the guy goes into the back storage room and a few minutes later comes out with a virtually new gear, I think he charged me $5
Check your local trans shops, walking in might be better than phone-calls
HIH
Mooser
3rd Gear
Joined: Sep 2012
Posts: 3
Likes: 0
Thanks,
I've contacted a few local, in other states, some GM on-line parts places, and a couple of specialty shops on-line. There are so many that I thought it might be good to ask for help. I'll keep checking the local shops: there are more. Most everywhere, the item is either discontinued or out of stock.
I've contacted a few local, in other states, some GM on-line parts places, and a couple of specialty shops on-line. There are so many that I thought it might be good to ask for help. I'll keep checking the local shops: there are more. Most everywhere, the item is either discontinued or out of stock.
Last edited by JamesPDX; Sep 20, 2012 at 09:26 PM.
